Thin Clients Booting over a Wireless Bridge

How quickly can thin clients boot over a wireless bridge, and how far apart can they really be?

In the 1970s and 1980s, the ubiquitous model of corporate and academic
computing was that of many users logging in remotely to a single server
to use a sliver of its precious processing time. With the cost of
semiconductors holding fast to Moore's Law in the subsequent decades,
however, the next advances in computing saw desktop computing become
the standard as it became more affordable.

Although the technology behind thin clients is not revolutionary, their
popularity has been on the increase recently. For many institutions
that rely on older, donated hardware, thin-client networks are
the only feasible way to provide users with access to relatively new
software. Their use also has flourished in the corporate context.
Thin-client networks provide cost-savings, ease network administration
and pose fewer security implications when the time comes to dispose of
them. Several computer manufacturers have leaped to stake their claim
on this expanding market: Dell and HP Compaq, among others, now offer
thin-client solutions to business clients.

And, of course, thin clients have a large following of hobbyists and
enthusiasts, who have used their size and flexibility to great effect in
countless home-brew projects. Software projects, such as the Etherboot Project and
the Linux Terminal Server Project, have large and active communities
and provide excellent support to those looking to experiment with
diskless workstations.

Connecting the thin clients to a server always has been done using
Ethernet; however, things are changing. Wireless technologies, such as Wi-Fi
(IEEE 802.11), have evolved tremendously and now can start to provide
an alternative means of connecting clients to servers. Furthermore,
wireless equipment enjoys world-wide acceptance, and compatible products
are readily available and very cheap.

In this article, we give a short description of the setup of a thin-client
network, as well as some of the tools we found to be useful in its
operation and administration. We also describe a test scenario
we set up, involving a thin-client network that spanned a wireless
bridge.

What Is a Thin Client?

A thin client is a computer with no local hard drive, which loads
its operating system at boot time from a boot server. It is designed
to process data independently, but relies solely on its server for
administration, applications and non-volatile storage.

Figure 1. LTSP Traffic Profile and Boot Sequence

Following the client's BIOS sequence, most machines with network-boot
capability will initiate a Preboot EXecution Environment (PXE), which will
pass system control to the local network adapter. Figure 1 illustrates
the traffic profile of the boot process and the various different stages, which are numbered 1 to 5. The network card broadcasts a DHCPDISCOVER
packet with special flags set, indicating that the sender is trying to
locate a valid boot server. A local PXE server will reply with a list of
valid boot servers. The client then chooses a server, requests the
name of the Linux kernel file from the server and initiates its transfer
using Trivial File Transfer Protocol (TFTP; stage 1). The client then loads and
executes the Linux kernel as normal (stage 2). A custom init program is then
run, which searches for a network card and uses DHCP to identify itself
on the network. Using Sun Microsystems' Network File System (NFS),
the thin client then mounts a directory tree located on the PXE server
as its own root filesystem (stage 3). Once the client has a non-volatile
root filesystem, it continues to load the rest of its operating system
environment (stage 4)—for example, it can mount a local filesystem and create a
ramdisk to store local copies of temporary files. The fifth stage in
the boot process is the initiation of the X Window System. This transfers
the keystrokes from the thin client to the server to be processed. The
server in return sends the graphical output to be displayed by the user
interface system (usually KDE or GNOME) on the thin client.

The X Display Manager Control Protocol (XDMCP) provides a layer of
abstraction between the hardware in a system and the output shown to the
user. This allows the user to be physically removed from the hardware
by, in this case, a Local Area Network. When the X Window System is
run on the thin client, it contacts the PXE server. This means the user
logs in to the thin client to get a session on the server.

In conventional fat-client environments, if a client opens a large file
from a network server, it must be transferred to the client over the
network. If the client saves the file, the file must be again transmitted
over the network. In the case of wireless networks, where bandwidth is
limited, fat client networks are highly inefficient. On the other hand,
with a thin-client network, if the user modifies the large file, only
mouse movement, keystrokes and screen updates are transmitted to and from
the thin client. This is a highly efficient means, and other examples, such
as ICA or NX, can consume as little as 5kbps bandwidth. This level of
traffic is suitable for transmitting over wireless links.